On
March 23, 2001, about 1115 Pacific standard time, a Republic RC-3
(Seabee), N6295K, sustained substantial damage subsequent to the
separation of the propeller while in cruise flight approximately 5
statute miles west of Tacoma Narrows Airport, Gig Harbor, Washington.
The airplane is owned by the pilot, and was being operated as a
personal/pleasure flight under the provisions of Title 14, CFR Part 91,
when the accident occurred. The commercial pilot, the sole occupant of
the airplane, was not injured. Visual meteorological conditions
prevailed and no flight plan had been filed. The flight originated from
Mason Lake, Washington, approximately 5 minutes prior to the accident.
The airplanes intended destination was Tacoma Narrows Airport, Gig
Harbor, Washington.

In a telephone conversation with the NTSB, the pilot stated that the
airplane was in a cruise flight configuration at 1,500 feet mean sea
level (msl) when he observed an engine over speed condition. The pilot
reduced engine RPM and executed a forced water landing near Carr Inlet.

Post-accident inspection of the airplane disclosed that the airplane's
propeller had separated from the crankshaft flange. The remnants of
three propeller bolts were recovered; however, the propeller assembly
was not recovered.

The recovered pieces of propeller bolts were submitted to the NTSB
Materials Laboratory in Washington, D.C. for metallurgical examination.
The Materials Laboratory's factual report (Report No. 01-076, attached)
noted that fractures in the bolts were found in two locations, adjacent
to the underside of the head and through the shank of the bolt. All head
fractures were noted to be 0.5 inch from the end of the bolt head. The
shank fractures in the two recovered bolt shank sections were noted to
be 1.0 and 1.1 inches from the end of the head, respectively.

The fractures adjacent to the heads of the three bolts were similar in
appearance, taking place on planes that initiated at opposite sides of
the bolt and were oriented at a slight angle towards the bolt head. Each
head fracture contain two smooth areas emanating from diametrically
opposite surfaces, which met at a rough area in the interior of the bolt
cross-section (typical of fatigue cracking followed by unstable crack
propagation due to overstress directly before complete fracture.) In all
three of the head fractures, about 80% of the fracture surface was found
to be characterized by fatigue. Scanning electron microscope (SEM)
examination of the head fracture surfaces disclosed that fatigue cracks
initiated in multiple points on diametrically opposite sides of the bolt
and propagated inwards.

Examination of the shank fractures also disclosed features consistent
with a fatigue cracking fracture mode followed by final fracture due to
tensile overstress. As with the head fractures, the primary features of
the shank fractures included a smooth (fatigue) area through
approximately 80% of the fracture surface, with fatigue cracks
originating from multiple locations on diametrically opposite sides of
the bolt and propagating towards the interior of the cross-section. SEM
examination of the shank fractures additionally detected micro-fissures
typically seen in high stress fatigue cracking in steel alloys.

Examination of the threads on the sides of the recovered shank pieces
were flattened. In each case, approximately 4 threads were flattened
over a length of about 0.2 inch.

Aircraft maintenance records indicate that the aircraft's propeller was
removed and replaced 2.8 flight hours prior to the accident. The
propeller was removed in order to comply with AD (Airworthiness
Directive) 97-18-02.